The present invention relates to an arrangement for feeding a coating mixture, size, or other treatment substance to an application device used to treat a moving paper or board web.
In coating or otherwise treating paper or board, the treatment substance is spread either directly onto the moving web or onto an applicator roll through a gap running across the machine or through several nozzles spread over the width of the machine. In short-dwell, jet, or spray coaters, the treatment substance is fed to an application chamber, or to a distribution chamber behind it, through a feed pipe intended to distribute the substance evenly to feed members, which even out the treatment substance and feed it as evenly as possible transversely over the surface of the moving web. Usually, the web is dosed with a considerable excess of treatment substance, compared to the desired final amount of substance, the excess being doctored off the surface of the web or roll after application. A considerable excess of treatment substance is fed to the application chamber, in which the pressure remains more or less constant over the entire width of the device, even though the web and application device may be very wide. Modern application devices usually feed the web with an amount of treatment substance significantly greater than the amount transferred to the web or roll and the pressure in the application chamber is not affected enough to create a pressure difference sufficient to prevent the application device""s leveling members or the next ones from giving the end product an even enough coating layer to satisfy customer requirements.
However, the development of treatment devices and increased quality demands from end-product users make greater demands on treatment substance feed methods. Flows in the application chamber must be controlled precisely and all turbulence must be eliminated as far as possible, to ensure, for instance, an even flow of treatment substance from the nozzle gap of a jet coater. Now it is necessary to try to keep the amount of coating as near as possible to the final amount of treatment substance, or even to dose the treatment substance to correspond exactly to the desired final amount of dry solids. This eliminates a return flow of treatment substance from leveling, thus avoiding or substantially reducing drying and dirtying of the treatment substance. An even pressure in the application chamber across the web is easier to maintain if large amounts of treatment substance are fed, but this requires larger feed channels and more powerful pumps to move the treatment substance.
On the other hand, paper grades and manufacturing methods have been developed, in which the paper is given an extremely thin coating layer. In these methods, coating amounts may be 1-5 g/m2, requiring a quite precise profile in the amount of treatment substance applied, as even small variations will cause a large relative variation between the actual and desired amounts of coating. Another problem is devices used to spread size on the web followed by a small amount of coating mixture with a high dry solids content. Film-transfer coaters are examples of such devices. In size and coating application devices without return circulation, for example spray and jet application devices, the feed amounts depend on consumption and are very much smaller than previously. Especially with viscous substances and at low speeds, the feed profile can easily acquire a slope, as the nozzles must be designed to the demands of the highest production speed. Thus, for example, the spray nozzles cannot be given a small enough distribution and size to create an even profile with even the smallest dose, as this would lead to the application chamber filling too slowly, the feed-line pressure increasing, peaks in the feed profile, and a greater danger of blockages in the small nozzles.
Publication WO 98/21408 discloses an arrangement for feeding a coating substance to the application chamber through two feed chambers. In this solution, two feed chambers are connected to the application chamber, each feed chamber being parallel to the application chamber and to the other feed chamber. Both feed chambers are connected to the application chamber through inlet openings, so that the flows from both chambers mix in the application chamber. Each feed chamber has its own treatment substance inlet and exit openings, arranged with the inlet opening of the first feed chamber next to the exit opening of the second feed chamber and vice versa. Thus, there is one inlet opening and one exit opening on each side of the coater and a through flow transverse to the web being coated travels through each feed chamber. In one embodiment, part of the treatment substance leaving the feed chamber can be led to the inlet opening of the adjacent feed chamber, to recirculate the treatment substance.
The arrangement described above can be used to create a more even pressure distribution in the application chamber and to reduce the excess pressure in the chamber, as the even pressure creates an even flow of treatment substance across the web. This solution has the problem of using two feed chambers, to which treatment substance is fed from both sides of the coater. As the use of two feed system and pumps is not economically viable, the treatment substance is fed to the feed chambers along lines from a single pump. Obviously it is then difficult to locate the feed lines and pumping equipment in relation to the manufacturing line. The inlet openings of both feed chambers must have the same pressure, to create an even pressure in the application chamber. Different feed pressures would lead to a danger of the application chamber having a lower pressure on the side with the lower feed pressure, resulting in a drop in the feed amount of treatment substance and a profile error. Thus, the flow resistance in the treatment substance feed piping should be equal in both feed chambers. Equal flow resistances are preferably created by using symmetrical pipe runs, but then the pumping equipment must be placed in the centre of the machine, which is often impossible. Besides, doubled piping becomes complicated and long, increasing equipment costs and the risk of disturbances. A particular problem is that the piping may become dirty, due to the treatment substance drying and particles of it building up in the piping and changing the flow resistance. Of course, this can be avoided by appropriate maintenance and sieving the circulating treatment substance properly.
The invention is intended to eliminate the defects of the state of the art disclosed above and for this purpose create an entirely new type of solution for feeding an application device with coating substance, size, or other treatment substance.
The invention is based on feeding the treatment substance to the application chamber of at least one application device, from at least one feed chamber with several connections opening to the application chamber and a flow route arranged to the opposite end of the feed chamber to the treatment substance inlet side, to connect the ends of the feed chamber, or a channel for returning at least part of the treatment substance to the end of the treatment substance inlet side of the first feed chamber of the application device.
According to one preferred embodiment of the invention, the return channel is a second feed chamber, connected to the application chamber and set parallel to a first application chamber and connected to a second application chamber, so that the treatment substance circulates from the first application chamber to the second application chamber, excess substance being removed from the second application chamber at the end on the treatment substance inlet side of the application device.
According to another embodiment of the invention, the return channel is a closed pipe, which connects the treatment substance exit side of the feed chamber to the inlet side, to even the pressure in the feed chamber.
More specifically, the arrangement according to the said invention is characterized by what is stated in the characterizing section of claim 1.
Considerable advantages are gained with the aid of the invention.
In the case of film-transfer coaters, which are made for spreading both size and coating mixtures, both the design of the film press and the number of the various alternative constructions in the product family can be reduced. As the invention can be used to give an even transverse application profile over the web independently of the substance to be applied or the feed amount, the same feed beam construction and dimensioning can be used for several machine widths, without having to dimension the feed system specially for each width manufactured. The system""s bypass line preferably returns the treatment substance to the inlet side, making the implementation of the treatment substance feed, return, and bypass lines and connections as simple as possible. If doctoring is used to level the treatment substance or if the application uses return circulation, the exit connections of the collector troughs can also be placed on the same side of the machine, so that, if desired, all the coating station""s connections can be on the same side of the machine. In jet and spray application devices, two sequential nozzles or rows of nozzles can be used, fed by means of circulation feed piping according to the invention, when the total application amount of the sequential nozzles will remain constant across the web. This solution has the further advantage that, if one nozzle becomes blocked, treatment substance will still be brought to the web, preventing untreated areas forming. As the transfer of the treatment substance to the web can be ensured, doctor or roll damage due to a dry web or applicator roll can be avoided.